The present invention relates to the testing of bituminous concrete (blacktop), and, in particular, to a method and apparatus for analyzing bituminous concrete mix to predict the probability of early rutting.
Today, highways and other roads are constructed from bituminous concrete. Bituminous concrete, which is commonly referred to as blacktop, is typically formed from a mix of different size aggregate materials. Because the properties of bituminous concrete will vary depending on the mix of aggregates that are used (or even available) to make it, the ability of one mix to endure traffic volume will often vary from that of another mix.
Today""s increased traffic volumes on highways and other roads require today""s road pavements to be better designed and have a longer life so that traffic can move along such roads safely and with little delay. This demand for better designed road pavements has increased the need to better predict pavement performance to avoid the need to prematurely replace pavements that fail under high traffic volumes. One indicator of a pavement""s ability to endure high traffic volumes is its ability to resist rutting caused by the traffic traveling over the pavement.
In more recent times, the demand for road pavements that are capable of enduring high traffic volumes has resulted in the development of what are known as Superpave mixes of blacktop. Superpave mixes are typically coarser mixes having better performance characteristics, vis-à-vis, rutting, fatigue cracking and low temperature cracking. Because the design of the aggregate mix used to make a Superpave mix can determine how a road paved from the mix will perform, there still exists the need to predict pavement performance with respect to a road""s ability to endure high traffic volumes.
One tool that has been used in the design of pavement mixes is the gyratory compactor, a device used to compress specimens of blacktop mixes for the purpose of evaluating the rate of densification of the mix. Typically a blacktop mix will include large and small aggregates. The kneading action provided by the gyratory compactor allows a specimen of the blacktop mix to be compacted. The mix is normally compacted 75, 115, 160 or 205 gyrations depending on the specifications and traffic demands for a particular application, as shown in Table 1 below.
After compacting the mixture, it is tested for density. A densification curve is calculated by comparing the specimen""s density with height measurements that are automatically recorded throughout the compaction process. This densification curve is then analyzed for compliance with specified density criteria (See Table 2 below). If the air void content is less than 2.0% at the specified maximum number of gyrations, the mix is deemed to be unacceptable.
The present invention is a method and apparatus for analyzing bituminous concrete mixes to predict the likelihood of early rutting. The present invention uses a FAT test head that is designed to be used in a gyratory compactor and that takes advantage of the kneading action provided by the gyratory compactor for the purpose of analyzing bituminous mixes to determine whether they are likely to prematurely rut.
According to the present invention, a test specimen of bituminous mix is prepared to a specified air void content and then heated to a constant temperature and then placed in a gyratory compactor. The FAT test head, which consists of a hard rubber block in the shape of a truncated cone mounted in the direct center of a flat, metal circular disk, is placed on top of the specimen in the gyratory compactor with the rubber head down and engaging the test specimen. The gyratory compactor is operated in the normal compaction mode. After the compaction is completed, mix specimen compaction data is analyzed to ascertain the degree of mix deformation for a specified number of gyrations, to thereby predict whether the mix is likely to prematurely rut. Thus, the present invention is useful in determining the best rut resistant aggregate mix when the aggregate is in the aggregate proportioning phase of a volumetric mix design.
The present invention and its advantages will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiment of the invention taken in conjunction with the accompanying drawings.